Theory of Operation
MAP sensors are three wire devices that measure intake manifold vacuum. In
actuality, the MAP sensor measures the difference between intake
manifold pressure/vacuum and atmospheric pressure. This is the reason why
intake vacuum
is NOT the same
as MAP vacuum. Intake vacuum is
atmospheric pressure minus MAP vacuum. With this in mind, the ECM makes the
appropriate calculations as to the correct injector pulse. The MAP scanner
PID is just MAP vacuum and should not be confused with intake vacuum. Few
manufacturers do put out a manifold vacuum PID and Chrysler is one of them.
In this case, the scanner PID for intake vacuum is a calculation
(atmospheric press. minus MAP vacuum).
An engine’s vacuum is a good indicator of load. The MAP
sensor outputs a DC voltage or frequency and its signal is inversely
proportional, which means that as manifold vacuum increases voltage or
frequency decreases. MAP sensors are also used as barometric (BARO) sensors.
As soon as the ignition key is turned on, the ECM reads the MAP voltage or
frequency signal and automatically takes that reading as atmospheric
pressure. Some manufacturers have configured their ECM programming so that
the barometric reading is updated during a WOT condition. Once the engine
starts, the ECM uses the MAP, TPS and RPM signals as main inputs to
calculate fuel control on
MAP or
SPEED DENSITY SYSTEMS (systems
without a MAF sensor). The ECM modifies injector pulse-width according to
the MAP signal output or engine load. This sensor is also used for ignition
timing and on some systems is a backup for the MAF sensor. With dual MAP and
MAF systems, the MAP sensor is primarily used to monitor the EGR valve
operation.
MAP sensors are made of a piezoelectric material. This
material is a form of crystal (Quartz) that when bent changes its internal
resistance. MAP sensors output two different types of signals. Most output a
voltage and usually work with a 5.00 Volt REF. The other type found mostly
on FORDs, output a square wave at a certain frequency (FORD uses 159 Hz). As
manifold vacuum increases the frequency output decreases.
CONDITIONS THAT AFFECT OPERATION
MAP sensors are connected directly to manifold vacuum. This
also means that any condition affecting the engine vacuum will also affect
the MAP sensor reading. Conditions that affect engine vacuum are:
EGR stuck open, clogged catalytic converter,
engine mechanical problems, vacuum leak, ignition timing problems, valve
timing adjustments and low fuel pressure.
Also a shorted sensor feeding off the same
sensor ground or 5.00 volt ref. line could cause a faulty MAP reading, due
to the bad sensor shorting the MAP signal.
NOTE: It is important to understand
that the MAP and RPM or Distributor Reference signals are the two most
important inputs to the ECM for fuel control. These signals should be
quickly assessed and analyzed when encountering any fuel control related
problems.
NOTE: Beware of the fact that on a
scan tool, some ECMs will substitute MAP reading on a rough running engine
if it sees the signal out of range. Therefore the scan tool reading may be a
substituted value that will throw the tech off during diagnostics.
COMPONENT TESTING
All MAP sensors have three electrical wires going to it:
sensor ground, reference voltage, and signal wire. The sensor ground is
provided by the ECM for all the other sensors. A ground voltage drop test
should be performed, between sensor ground and battery post ground to verify
no more that a 100 mV voltage drop during KOER.
The reference voltage is also provided by the ECM and is a
5.00 volts regulated feed line. It provides the MAP sensor with its working
voltage. A shorted 5 volt reference line, either the line wire or another
sensor that is shorting it will directly affect the MAP sensor reading and
therefore the entire engine.
The signal line is the signal return to the ECM. It provides
the ECM with the actual MAP sensor reading. This is the line to tap to when
performing actual tests and comparing them to the tables found here. A
shorted MAP signal line will also adversely affect the sensor’s reading.
• The first step in analyzing a MAP sensor is the KOEO
signal reading. If this reading is wrong to begin with, the ECM will react
as if the vehicle is operating at higher altitude and the engine will run
richer, since we need more fuel at high altitudes.
• Second, make sure that the scan reading is the same as the
actual reading coming from the sensor itself. This will verify that the ECM
is not substituting the signal values.
• Third, make a careful analysis using the “conditions
that affect the MAP sensor” to find any causes that may be inducing a
problem.
• Last, there is always the possibility that the ECM is at
fault. However, a defective ECM will almost surely be in the sensor ground
or 5 volt reference line and will probably affect other sensors. This
sequential diagnostic analysis will lead you in the right direction.
By following these simple steps and carefully analyzing the
corresponding wiring diagrams (s), it will surely point you in the right
direction. If the reference voltage is gone, DO NOT assume that ECM
is defective. If one of the sensors tied to the same reference line is
shorted, it will bring down the reference line and all sensors connected to
it. So, check your reference voltage.
